1. Field of the Invention
The present invention relates to an optical element capable of changing its focal length and to an image taking apparatus which takes an image of a subject.
2. Description of the Related Art
As a variable-focus lens capable of changing the focal length, a liquid crystal lens capable of changing the focal length by using the electro-optic effect of a liquid crystal is known. For example, Japanese Patent Laid-Open No. 2002-341311 (patent document 1) discloses a liquid crystal lens having first and second light-transmissive substrates in the form of a flat plate, a third light-transmissive substrate having two concave surfaces and provided between the first and second light-transmissive substrates, and a liquid crystal enclosed in each of a space between the first and third light-transmissive substrates and a space between the second and third light-transmissive substrates. In this liquid crystal lens, the orientation of liquid crystal molecules is changed according to the level of an applied voltage to change the refractive index of the liquid crystal lens. The focal length of the lens is thereby changed.
A fluid lens which can be changed in shape by application of a voltage to change its focal length is also known as a variable-focus lens. For example, a fluid lens in which an immiscible fluid constituted of a non-electroconductive oil and an electroconductive aqueous solution is enclosed in a tube having its inner wall surface covered with a water-repellent coating is proposed in “Philips' Fluid Lenses”, [online], Mar. 3, 2004, Royal Philips Electronics, [found on Mar. 31, 2004], Internet <URL: http://www.dpreview.com/news/0403/04030302philipsfluidle ns.asp> (non-patent document 1). In this fluid lens, when no voltage is applied, the aqueous solution constituting the immiscible fluid is a semispherical mass and the interface of the aqueous solution on the oil is convex. This interface changes between the convex state and a concave state according to the level of the applied voltage. Consequently, the radius of curvature of the lens can be changed and the focal length of the lens is freely variable.
In the technique disclosed in the patent document 1, the focal length of the lens is changed by using the difference Δn (n∥−n⊥) between the refractive index (n∥) in the major-axis direction and the refractive index (n⊥) in the minor-axis direction of liquid crystal molecules. However, there is a problem that the difference Δn is so small that the refractive index of the lens cannot be freely changed.
In the technique proposed in the non-patent document 1, the focal length of the fluid lens is changed by applying a voltage to the immiscible fluid. When a voltage is applied to the immiscible fluid, a current flows through the aqueous solution constituting the immiscible fluid. Therefore, there is a risk of the aqueous solution being decomposed by electrolysis to generate hydrogen and oxygen, and there is a problem that during use over a long time period a gas constituted of generated hydrogen and oxygen is accumulated to form bubbles which scatter light and deteriorate the performance of the lens.
The above-described problems arise generally with optical elements such as plane-parallel plates and prisms as well as with lenses.